1. Field of the Invention
The present invention relates to a recording sheet and a color developer therefor, and more particularly to a recording sheet having coated thereon a novel color developer layer and a novel color developer for the recording sheet.
2. Description of the Prior Art
There have been known recording sheets utilizing a color reaction of a colorless organic compound (hereinafter referred to as a "color former"), and an adsorbent material capable of forming a distinct color when contacted with the color former.
As the recording sheets using the phenomenon, a pressure-sensitive recording sheet (U.S. Pat. Nos. 2,505,470, 2,505,489, 2,550,471, 2,548,366, 2,712,507, 2,730,456, 2,730,457, 3,418,250, etc.) and a heat-sensitive recording sheet have been well known. Further, a printing method has been known wherein an ink containing the color former is applied to the color developer sheet either through a medium such as a stencil or not.
In all cases, the above phenomenon of the color reaction between the color former and the color developer are utilized under pressure with a pen or stylus, under heat, etc.
The color former may be dissolved in a solvent such as chlorinated diphenyl, chlorinated paraffin or other organic solvent. The solution may be dispersed in a binder and/or may be microencapsulated, and then coated on a support such as paper, plastic film, resin-coated paper, etc.
In the case of a heat-sensitive recording sheet, the color former may be coated on the support together with a thermofusible material such as acetoanilide which is melted under heating to dissolve the color former.
On the other hand, the color developer is dissolved or dispersed in water or an organic solvent together with a binder such as styrene-butadiene rubber latex and then coated on or impregnated into a base support before or directly before recording.
The color former and color developer may be coated on the same surface or opposite surfaces of a support, or on different supports.
Usually, after the color developer is coated on the support, a desensitizer is locally coated thereon in order that unnecessary color formation on certain surface portions is prevented (U.S. Pat. No. 2,777,780). Another embodiment of the prevention of color formation is to locally coat the color developer on the support or the coated layer of color former without using the desensitizer.
As the above described color developer, clays such as acid clay, active clay, attapulgite, zeolite, bentonite, etc.; phenol resins (U.S. Pat. Nos. 3,516,845, 3,540,911); and organic compouunds such as succinic acid, tannic acid, gallic acid or phenol compounds (U.S. Pat. No. 3,244,548) are known. Such organic compounds are not practically employed for the reason that the color developing ability of the color developer sheet (which is a support having coated thereon the color developer) is very low and the color developed on the sheet is low in light resistance although the compound can be very easily handled in manufacturing the color developer sheet.
Further, in the case where the above color developer is locally coated on a support to form a color developer sheet, even if a large amount thereof (7-10 g per 1 m.sup.2 of the support) is coated thereon, the color developer sheet cannot form a sufficient color when contacted with the color former. As a result, the local coating method is limited in utility.
In this case, since the locally-coated portion of the color developer sheet is thicker than the other portions, the color former coated on another support, when contacted with the color developer sheet, is locally pressurized to form unpreferable color (hereinafter referred to "fog").